Reflective display panels for portable applications have significant advantages over light emissive and transmissive devices. They are lighter, smaller, more power efficient, less irritating to the human eye, and function best under bright light—in comparison to light emissive or transmissive displays, which tend to be obscured by bright ambient lighting, such as direct sunlight. In recent years, extensive effort has been put into the development of reflective display technologies.
The most common technique seen in the market today centers around the electrophoretic effect, which operates on the principle that electrically charged particles will migrate away from surfaces charged to the same polarity as themselves and towards surfaces charged to an opposite polarity. Display devices which use this effect are known as electrophoretic image displays (EPIDs). Many patents, including U.S. Pat. Nos. 4,655,897; 4,732,830; 4,742,345; 4,746,917; 4,772,820; 5,360,689; and 7,259,744, describe and illustrate methods, and techniques for making EPIDs and the apparatus comprising them. While the low power consumption rate of EPIDs is a clear advantage, these displays exhibit extremely slow response times, making them unsuitable for video displays. Furthermore, the high cost of manufacturing color EPIDs presents another significant barrier to their widespread use for displays.
Other techniques are used in electrowetting displays and cholesteric liquid crystal displays, which are respectively described and illustrated in U.S. Pat. No. 6,911,132 (Pamula et al.), and U.S. Pat. No. 5,570,216 (Lu et al.). Electrowetting technology relies upon adjusting an electric field to modify the wetting properties of a hydrophobic surface. This method is effective in creating high-brightness, high-contrast screens. The quick rate at which the voltage can be switched in an electrowetting display partly overcomes the problem of slow response rate in electrophoresis devices.
The bistable cholesteric liquid crystal display mechanism takes advantage of light diffraction. Like electrophoresis screens, however, cholesteric LCD screens also respond too slowly to be suitable for video displays.
For use in reflective display technologies, techniques relying on light interference may be superior to the techniques described above, because of the relative simplicity of such devices. Electrophoretic image displays, electrowetting technology, and cholesteric LCDs each rely on the unique characteristics of specific materials, which tends to add to their manufacturing costs, since such materials are not readily available. Also, the display screens for such devices require interlaced sub-pixels, and consequently, have more difficulty in achieving a full color display. By comparison, optical interference display modules rely only on the mechanical properties of materials and are therefore the most promising of developing display techniques.
U.S. Pat. No. 5,835,255 (Miles) and subsequent patents disclose how light in the visible spectrum can be controlled with an array of modulation elements to make a display panel. As shown in FIGS. 1A and 1B (Prior Art), each modulator 100 contains two parallel reflective mirrors 102 and 104, which are separated by a cavity 101 and a transparent film 105. Standoffs 108 support mirrors 102 and 104 so that they are spaced apart from each other, defining cavity 101. Electrodes are provided on each of the mirrors. Mirror 102, which is the first of the two mirrors to interact with incident light, partially reflects the incident light, but transmits the remainder of the incident light. Mirror 104 then reflects the light that has passed through mirror 102. When a voltage is applied to the electrodes on each mirror, mirror 104 is drawn toward mirror 102, collapsing the cavity, and switching the modulator from its natural “ON” state (as shown in FIG. 1A) to an “OFF” state (shown in FIG. 1B). Switching to the OFF state effectively eliminates cavity 101, so that mirror 104 is collapsed onto transparent film 105, which is an electrical insulator. Transparent film 105 thus prevents electrical current from flowing between the two mirrors and determines the spacing between the two mirrors in the OFF state. The optical property of modulator 100 in its OFF state depends on the thickness of the transparent film. Adjusting the spacing between the two mirrors in the ON state of the modulator can alter the phase of the light components, to change between constructive and destructive interference. Therefore, switching modulator 100 from ON to OFF changes the pixel from a primary color (where the specific color depends on the spacing between mirrors 102 and 104), to black. In one embodiment, provision of a color display using this technique requires at least three sub-pixels, each sub-pixel representing a different one of the three primary colors for a color pixel. When the voltage is removed from the electrodes on mirrors 102 and 104, modulator 100 should reliably switch from the ON state to the OFF state. This requirement can present a challenge to the manufacturing process due to the tendency of contact parts in micro structures to exhibit static friction, which is referred to as “stiction.” Anti-stiction bumps are usually used to improve the reliability of the release operation between contacting parts in such structures. Some designs in the prior art use tethers that cause mirror 104 to twist to release the tensile, or compressive tension, and also to help to reduce stiction between the mirror and the transparent film.
Three plate structures have also been proposed in the prior art to achieve multistate color modulation (as disclosed in U.S. Pat. No. 7,372,613). In this design, an additional plate with electrodes is attached to the structure. The movable plate is therefore tri-stable and can toggle between three states.
Considerations have to be given to the balance between the elastic force for releasing the mirror from the stiction forces, and the driving voltage. On one hand, the elastic restoring force is helpful to reduce stiction. But to drive the movable mirror, an undesirably high electrostatic force is required (i.e., higher voltage). It is difficult to find the appropriate balance, especially when the requirements of restoring and driving force for different colors are different.
Despite any advantages, the aforementioned techniques all share a common problem—a requirement for extremely precise structures that are both complex and costly to fabricate. The techniques all use spatial dithering to maximize a display screen's resolution and color depth. Spatial dithering requires that every pixel unit include a minimum of three sub-pixel modulators disposed side-by-side, and each modulator for the three or more sub-pixel colors must be manufactured differently by applying distinctive steps. Activation of every sub-pixel must also be controlled through row and column lines. Precision is crucially important because the sub-pixels are small and the external electrical connection structure must be complex. It should be evident that such precision is difficult and costly to achieve.
U.S. Pat. No. 7,006,272 discloses a method that attempts to avoid this problem by modulating a light beam so as to realize color changeable pixels. The principle of color modulation is similar to that disclosed in U.S. Pat. No. 5,835,255, but the method instead uses a three-parallel-plate structure (see prior art FIGS. 2A-2C). A bottom plate 202 and a middle plate 204 are both reflective and form a cavity. Middle plate 204, however, is deformable and its vertical position is controllable by applying a voltage to either plate 202 or a top plate 206. Adjusting the voltage moves middle plate 204 by a desired extent, thereby altering the frequency of the reflected light produced by constructive interference. However, this patent discloses no specific mechanisms that achieve a low curvature of the middle plate when actuated with the applied voltage. Theoretically, middle plate 204, if it is uniform in thickness, would deflect in response to the applied voltage to form a hyperbolic surface, as shown in FIG. 2C, unless the plate deforms sufficiently to touch bottom plate 202 (and flattening like mirror 104 does when it contacts transparent film 105, as shown in FIG. 1B). Therefore, a specific patterned or non-uniform middle plate configuration is required to structurally keep the plate flat. The use of such free moving plates has been attempted, but has not yet been shown to be successful.
To ensure that middle plate 204 moves freely, while retaining a flat shape, four tethers (or at least three) have been used in the prior art, with a tether being disposed at each of the corners of the moving middle plate, to control the deformation of the middle plate and keep the middle plate flat when the voltage is applied to move the plate (e.g., see the disclosure of U.S. Pat. No. 5,999,303). Beam structures have also been used in prior art tunable mirror applications (see for example, the disclosure of U.S. Pat. No. 5,312,513). The structure of the plate and tethers/beams are deposited on a sacrificial layer through sputtering and then patterned using conventional lithography techniques. The sacrificial layer is then etched away, leaving the structure suspended by supporting posts. Upon release, tethers and beams typically experience deformation due to residual stress, which causes the tethers/beams to shrink or expand. In addition, due to uncontrollable manufacturing deviations, the tethers/beams may not be consistent from one fabricated device to the next. These variations can cause poor color realization or malfunction. A few faulty pixels in a display panel can result in the entire panel being discarded—which can substantially increase fabrication costs.
Display products conventionally use red, green, and blue (RGB) to form color pixels. Different proportional combinations of red, green, and blue components can be used to produce the full spectrum of colors. Printers, on the other hand, generally use a subtractive color mixing method to produce colors. In particular, varying amounts of cyan, magenta, yellow, and black (CMYK) inks are applied to a sheet of white paper in layers. Each layer subtracts some of the light that would otherwise be reflected from the white background, and various combinations of the layers can provide a particular color. By using different proportional amounts of the tinted inks in the layers, almost any visible colors in the full spectrum of colors can be produced. Thus, for printed images, colors are produced on a page by subtracting selected proportions of different wavelengths of light from the light that would be reflected from a white background.
It is also conventional to use RGB primary colors to produce the variety of colors created on emissive or transmissive display devices. It appears to be practical and economical to do so, since single-bandwidth light is more convenient to obtain through either light emission or color filters. Moreover, black and white can be realized on a black background in an RGB display, without adding an extra (i.e., a fourth) color dimension.
But color conversion from RGB used for display products to CMYK that is generally used for printers decreases color fidelity. U.S. Pat. No. 7,586,472 (Marcu et al.) teaches a display using a color subtractive method to address the problem of achieving a full color display, for light transmissive devices.
In practice, the significant number of pixels on a display panel required to achieve a desired resolution makes the panel difficult to manufacture. Any malfunctioning pixels compromise the quality of the overall panel, so that a panel with more than a few defective pixels is not usable. Too many defective panels result in an unacceptable low production yield that increases fabrication costs. Thus, it remains difficult to produce display panels of a consistent quality at reasonable cost level.
Thus, there is clearly a need for a color display panel that uses a reflective light modulation array, and which is not only readily implementable, but also fast in response, low in power consumption, low in cost, and easy to make. It is preferable to provide colors that are modulated in CMYK using the subtractive method even though RGB additive method has conventionally been adopted in other types of panel displays. The requirement for fabrication precision when producing complex structures should be transferred from the process used to manufacture the display panel to driver electronics that are external to the panel, to improve production yield and lower the cost.